1. Field of the Invention
The present invention relates to a sheet processing apparatus provided with a sheet sensor, and more specifically, to a sheet processing apparatus provided with, for example, sheet stack means on which discharged sheets are sequentially stacked and to an image forming apparatus provided with the sheet processing apparatus.
2. Related Background Art
A sheet processing apparatus arranged such that the upper surface of sheets stacked on a stack tray is set to a prescribed height at all times is known.
The sheet processing apparatus comprises a discharged sheet tray on which discharged sheets are sequentially stacked, lifting/lowering device for lifting and lowering the discharged sheet tray, an upper surface sensor for sensing the upper surface of the uppermost sheet of the sheets stacked on the discharged sheet tray and a control for controlling the lifting/lowering device based on a result sensed by the upper surface sensor. A light transparent type sensor, for example, is used as the upper surface sensor. The sensor is composed of a light emitting unit and a light receiving unit disposed on the right side and the left side of the sheet discharge tray, respectively, and the optical axis of them travels a predetermined height above the discharged sheet tray in a right and left direction.
Each time a sheet is stacked on the stack tray, the height of the uppermost sheet is increased. When the uppermost sheet reaches the optical axis, the emitted light is blocked by the sheet, that is, the sensor senses the uppermost sheet. The control lowers the stack tray by controlling the lifting/lowering device based on the result sensed by the sensor. A lowering amount of the stack tray at the time is set to an amount necessary to restore the optical axis shaded by the uppermost sheet. The repetition of the above operation effected each time a sheet is discharged onto the stack tray and stacked thereon permits the uppermost sheet of the sheets on the stack tray to be maintained to the prescribed height at all times.
With this operation, since the height from a discharge port from which a sheet is discharged to the uppermost sheet, that is, a falling height of a sheet when it is discharged can be maintained to the prescribed height, sheets can be discharged and stacked well.
However, according to the above prior art, when sheets discharged onto the stack tray are partially drawn out in a batch, the position of the uppermost sheet on the stack tray is lowered and the falling height of a sheet is increased when it is discharged. Thus, there is a possibility that sheets are discharged and stacked badly.